Re: [PATCH v3 3/3] ovl: get_acl: Fix null pointer dereference at realinode in rcu-walk mode

[Amir Goldstein]

On Tue, May 16, 2023 at 5:19 PM Zhihao Cheng <chengzhihao1@xxxxxxxxxx> wrote:
>
> Following process:
>          P1                     P2
>  path_openat
>   link_path_walk
>    may_lookup
>     inode_permission(rcu)
>      ovl_permission
>       acl_permission_check
>        check_acl
>         get_cached_acl_rcu
>          ovl_get_inode_acl
>           realinode = ovl_inode_real(ovl_inode)
>                               drop_cache
>                                __dentry_kill(ovl_dentry)
>                                 iput(ovl_inode)
>                                  ovl_destroy_inode(ovl_inode)
>                                   dput(oi->__upperdentry)
>                                    dentry_kill(upperdentry)
>                                     dentry_unlink_inode
>                                      upperdentry->d_inode = NULL
>             ovl_inode_upper
>              upperdentry = ovl_i_dentry_upper(ovl_inode)
>              d_inode(upperdentry) // returns NULL
>           IS_POSIXACL(realinode) // NULL pointer dereference
> , will trigger an null pointer dereference at realinode:
>   [  205.472797] BUG: kernel NULL pointer dereference, address:
>                  0000000000000028
>   [  205.476701] CPU: 2 PID: 2713 Comm: ls Not tainted
>                  6.3.0-12064-g2edfa098e750-dirty #1216
>   [  205.478754] RIP: 0010:do_ovl_get_acl+0x5d/0x300
>   [  205.489584] Call Trace:
>   [  205.489812]  <TASK>
>   [  205.490014]  ovl_get_inode_acl+0x26/0x30
>   [  205.490466]  get_cached_acl_rcu+0x61/0xa0
>   [  205.490908]  generic_permission+0x1bf/0x4e0
>   [  205.491447]  ovl_permission+0x79/0x1b0
>   [  205.491917]  inode_permission+0x15e/0x2c0
>   [  205.492425]  link_path_walk+0x115/0x550
>   [  205.493311]  path_lookupat.isra.0+0xb2/0x200
>   [  205.493803]  filename_lookup+0xda/0x240
>   [  205.495747]  vfs_fstatat+0x7b/0xb0
>
> Fetch a reproducer in [Link].
>
> Use the helper ovl_i_path_realinode() to get realinode and then do
> non-nullptr checking.
>
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=217404
> Fixes: 332f606b32b6 ("ovl: enable RCU'd ->get_acl()")
> Signed-off-by: Zhihao Cheng <chengzhihao1@xxxxxxxxxx>
> Suggested-by: Christian Brauner <brauner@xxxxxxxxxx>
> Suggested-by: Amir Goldstein <amir73il@xxxxxxxxx>

Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx>

> ---
>  fs/overlayfs/inode.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
> index ca56b1328a2c..e7e888dea634 100644
> --- a/fs/overlayfs/inode.c
> +++ b/fs/overlayfs/inode.c
> @@ -558,20 +558,20 @@ struct posix_acl *do_ovl_get_acl(struct mnt_idmap *idmap,
>                                  struct inode *inode, int type,
>                                  bool rcu, bool noperm)
>  {
> -       struct inode *realinode = ovl_inode_real(inode);
> +       struct inode *realinode;
>         struct posix_acl *acl;
>         struct path realpath;
>
> -       if (!IS_POSIXACL(realinode))
> -               return NULL;
> -
>         /* Careful in RCU walk mode */
> -       ovl_i_path_real(inode, &realpath);
> -       if (!realpath.dentry) {
> +       realinode = ovl_i_path_real(inode, &realpath);
> +       if (!realinode) {
>                 WARN_ON(!rcu);
>                 return ERR_PTR(-ECHILD);
>         }
>
> +       if (!IS_POSIXACL(realinode))
> +               return NULL;
> +
>         if (rcu) {
>                 /*
>                  * If the layer is idmapped drop out of RCU path walk
> --
> 2.39.2
>